Exhaust systems for motor vehicles are increasingly of the "catalytic converter" type. The function of the catalytic converter is to reduce the quantity of polluting gases emitted by the engine.
Catalytic converters contain a support material, generally alumina, onto which the noble metals which function as catalysts are deposited. In order to function correctly, the operating temperature of the catalytic converter must be sufficiently high. Motor vehicle manufacturers therefore tend to locate the catalytic converter as close as possible to the exhaust manifold so that the catalytic converter warms up as quickly as possible when the engine is started from cold.
On the other hand, excessively high operating temperatures reduce the service life of a catalytic converter by irreversible transformation of the support material. Such temperatures can also damage the entire exhaust system and necessitate the use of stronger and therefore more costly materials for each component of the exhaust system (muffler, exhaust pipes, etc). Such temperatures can be reached when the engine is working hard, for example on a freeway.
What is more, the improved thermal efficiency of internal combustion engines, in particular fuel injected diesel or petrol engines, reduces heat losses to the cooling circuit. The heat required to heat the passenger compartment is taken from the cooling circuit. Improved efficiency can therefore lead to insufficient heating.
Finally, the necessity to reduce polluting emissions from internal combustion engines has led to the use of new types of catalytic converter, one feature of which is that they operate within a narrow range of temperatures.
The present invention aims to mitigate these drawbacks. More precisely, the invention aims to limit or to regulate the temperature of the gases entering the catalytic converter whilst enabling the catalytic conventer to warm up quickly. The invention also aims to improve the heating of the passenger compartment of the vehicle and the conversion rate of the catalytic converter.